Study On The Flow And Sediment Characteristics And Regulation Of The Branching Channel In Fluctuating Backwater Area Of Reservoir

The branching channel in fluctuating backwater area of reservoir is not only influenced by the flow and sediment of the upstream,but also influenced by the operation of the downstream reservoir.This makes the river bed in the process of adjusting the new balance for a long time.And the branching channel is composed of two or more branches.The change of any branch will affect the whole branching system,making the water and sediment characteristics more complicated than the single channel.The combined action of various factors has created the complex water and sediment environment of the branching channel in fluctuating backwater area of reservoir,which will inevitably cause the river bed form readjustment and the change of its branching pattern,thus affecting the safety and benefits of navigation,flood control and fording projects.Therefore,accurately understanding and grasping the characteristics of water and sediment in such a river can lay a good foundation for the analysis of the same type river bed evolution law and the research of channel regulation engineering scheme,and has important theoretical significance and practical significance.In this paper,the Xiaonhai-Zhajiao reach is taken as the research object and the two dimensional mathematical model is used to analyze the change of water and sediment characteristics of the branching channel in fluctuating backwater area of reservoir.Only in the low water period,the reach is affected by the backwater of the reservoir,and the water level is generally raised,but the ascending range is not more than 3m,and the flow conditions in the navigation channel are greatly improved.Under natural conditions,the hydrodynamic axis and sediment transport path of the Xiaonhai river reach are stable at different discharges,and the transverse ratio of the diverging area increase with the increase of the volume rate.The hydrodynamic axis and sediment transport path's change of the Zhajiao river reach is large,they swing to the left bank as the flow increases,and the transverse ratio of the diverging area increase as the flow decreases.At the same time,with the aid of a three-dimensional mathematical model of generalized branching channel,the paper analyzes the flow structure characteristics of the branch from the space and the influence of the width ratio of the river branchs and the water depth controlled by the lower reaches on the spatial flow distribution characteristics.Each branch of a branching channel is equivalent to a curved channel.Due to the effect of the centrifugal force,the river section will form a circulation.The width ratio of the river branchs affects the transverse gradient of diverging area,and then affects the development of the transverse flow in the entrance area.When the width ratio of the river branchs increases,the transverse flow is enhanced.The controlling water depth of downstream affects the development of the circulation of branching channel.Only when the water depth is suitable,the circulation is fully developed,which is beneficial to the longitudinal and lateral sediment transport of the branching channel.On the basis of preliminary understanding of the above characteristics of water and sediment,a regulation method for the rapids of Zhajiao beach is put forward,and two dimensional sediment mathematical model is applied to predict the scouring and silting changes of the river section 10 years later.The results show that the channel area is in micro erosion state,the backwater region of the right branch and the rear of the delta has little sedimentation,the stability of the river bed is better whole after the implementation of the regulation scheme.